U-M Astronomers have front-row seats as our galaxy’s central black hole prepares to feast on a giant gas cloud.

Black hole expert and Hubble Postdoctoral Fellow Nathalie Degenaar and her U-M colleagues are preparing for what they hope will be a first-ever opportunity to watch our galaxy’s central supermassive black hole gobble up a giant gas cloud.

A simulation of gas cloud “G2” approaching the supermassive black hole at the center of the Milky Way. The cloud's orbit is marked in red, and the stars orbiting the black hole are shown with blue lines. This view simulates the expected positions of the stars and gas cloud in the year 2021. Credit: ESO/MPE/Marc Schartmann.

If the cloud indeed becomes a meal, it may generate an X-ray “light show” for years to come, providing rare insight into this black hole’s mysterious properties.

“The unusual thing about the Milky Way’s central black hole, which we call Sagittarius A* (Sgr A*),” says Degenaar, “is that it is extremely dim, much dimmer than you would expect for its size. It seems as though it’s not eating much. But if it consumes this gas cloud, it will give us a unique opportunity to observe in real time how our black hole eats and what kind of radiation it generates – important physical processes that we don’t yet fully understand.”

This is important because even though dim black holes may lurk at the center of galaxies all around us, they are too faint and distant for us to study. So we need to learn about their behavior by studying the one in our own backyard.

This much-anticipated “dinner date” is the perfect opportunity. However, it is by no means a sure thing; if the cloud is hiding something heavy like a star, it may largely escape the black hole’s gravitational pull. But if it does happen, Degenaar and her U-M colleagues will have front-row seats. She serves as the principal investigator on the Swift galactic center campaign, which has been pointing NASA’s Swift X-ray telescope at the Milky Way’s core for 17 minutes every few days since 2006. Degenaar’s been there from the start.

The campaign has long been recording X-ray flares from Sgr A*, during which itbecomes as much as 150 times brighter (see video here). This happens for a few hours every week or so, and may be the result of the black hole swallowing a small astronomical body like a comet or asteroid.

If the gas cloud is ingested, optimistic estimates are that the flares may be as much as 10,000 times brighter and last for years, if not decades, says Degenaar. Her role with Swift is allowing her to both promote the event among fellow astronomers and hit the ground running with analysis as data becomes available.

“One of the great things about Swift is that the data is public right away – generally within just two to three hours after the actual observation, which is incredibly fast,” says Degenaar. However, astronomers would have to process the raw data themselves to see if the black hole’s behavior was changing.

To streamline this process and provide user-friendly information, Degenaar worked with two U-M colleagues, Associate Professor Jon Miller and Assistant Research Scientist Mark Reynolds, to create a public monitoring website. It features a robust data reduction pipeline developed by Reynolds, which creates images from each observation and updates the long-term light curve so astronomers know immediately if the black hole has brightened. Visitors can even sign up for email alerts when flaring behavior is detected. The aim is that researchers observing the event in other wavelengths can quickly turn their telescopes toward the galactic center should the feasting begin.

To ensure this happens, the Swift program has beefed up its team to provide “expertise that never sleeps.”

“We have a number of team members in different parts of the world,” says Degenaar, “so there are always eyes on the galactic center. If it’s night in Europe, we’re at watch in the U.S. If it’s night here, researchers in Europe are keeping an eye out.”

The team includes members who have been with the campaign since its beginning like Degenaar and her former PhD advisor, Rudy Wijnands from the University of Amsterdam in the Netherlands, both of whom are experts on black holes and neutron stars, along with Jamie Kennea from Penn State, who is part of the Swift instrument team. It also includes new members brought in specifically to study this event: Mark Reynolds from U-M, who set up the website and data pipeline; Jon Miller from U-M and Sera Markoff from the University of Amsterdam, who are both experts in black hole accretion and can help explain the observations; and two astronomers with complementary observing programs, Daryl Haggard from Northwestern University (radio telescopes and Chandra X-ray satellite) and Gabriele Ponti of the Max Planck Institute for Extraterrestrial Physics in Germany (XMM X-ray satellite).

Being at the center of this international team of experts has positioned Degenaar and her U-M colleagues ideally for this event. “We’ve been looking at the behavior of this black hole in X-ray for years,” she says, “and we’re experts in analyzing the data. Together with our colleagues, we’re in a strong position to assess whether something is changing and what it might mean. If the flares brighten or happen with different frequency, for example, this might show us something about the mechanism driving them. Other changes in X-ray emission may tell us about the way in which matter flows toward the black hole – the trajectory it’s taking.”

But as exciting as this opportunity is for Degenaar, she’s quick to point out that Swift’s galactic center campaign offers more than just a seat at Sgr A*’s dining table. “The program has an incredibly rich data set,” she says. “The center of our galaxy harbors a large concentration of stars; star clusters; and compact objects like white dwarfs, neutron stars, and black holes – so there are many other interesting things to study.”

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